Hitherto, it has been proposed to select narrow pass bands from a relatively broad band optical source such as an optical laser, using Bragg-matched reflection gratings, for example recorded in an optical fibre. For a general review, reference is directed to Photosensitive Optical Fibres: Devices and Applications, R. Kashyap, Optical Fibre Technology 1, 17-34 1994. As explained in this review paper, band pass filters can be constructed for filtering out several wavelengths in a narrow range of a few nanometers based on a Michelson Interferometer as proposed by D. C. Johnson et al, "New Design Configuration for a Narrow Band Wavelength Selective Optical Tap and Combiner" Electron. Lett. Vol. 23, 668 (1987). In this configuration, a pair of Bragg matched reflection gratings are carefully positioned in the arms of the Michelson Interferometer so that the optical path lengths to and from the gratings are identical. Thus, the device is carefully tuned in terms of the matching of the gratings and the path length.
In WDM bandpass filters, it is usually necessary to select a number of different pass bands centred on different wavelengths. Hitherto, it has been proposed to include a number of pairs of identical gratings in each arm of the interferometer, as discussed by Kashyap, supra. However, the resulting structure is inflexible because the wavelengths of the pass bands are fixed by the characteristics of the individual pairs of tuned gratings, and the device is difficult to set up for use with desired pass bands.